Autocorrelation and Autospectra Estimation of Reciprocating Engine Turbulence

A general method for analyzing the time-correlation and frequency-spectral structure of turbulence in IC reciprocating engines was developed and applied to the cycle-resolved turbulent velocity fluctuation and to the fluctuating motion in its more conventional sense. It is based on an alternative definition of the Eulerian temporal autocorrelation coefficient so as to reduce this to an even function solely of the separation time within specific correlation periods into which the engine cycle is divided. This procedure was shown to be a refined version of a previous approach to spectral analysis of engine turbulence used by the authors, and was compared to the one based on the rough application of two definitions of nonstationary autocorrelation coefficients in the standard approach for stationary flows. It was proved to be suitable for studying the average statistical properties of segmented nonstationary turbulence sample records, being easily extensible from temporal to spatial records. The method was applied to the analysis of the effects of different swirl flow conditions on correlation and spectral turbulence quantities in an automotive engine. The spatial distribution of the in-cylinder dissipation time scale of turbulence and, in particular, the influence of the wall on this parameter were also investigated.